This whole movie was made to flesh out his character, so of course he would get most of the spot light. But I agree, no Genin has a place in a fight between demi-gods and Otsustuki. They forced Boruto in there and as a result Naruto and Sasuke got heavily nerfed so that Boruto would get an opportunity to shine.
As for Sasuke mentoring Boruto, it was done for symbolism and to parallel Sarada's situation with Naruto. Both Naruto and Sasuke helped each other's children to understand their fathers and get closer to them, and it resulted in Boruto wanting to become a Shinobi like Sasuke just like Sarada wanting to become a Hokage like Naruto. It's a way to form a connection between those 2 families.
If kishi could drop a shirt 10 chapters of Sakura traveling with Sasuke, maybe, just maybe I'll get over it.
He should of added that towards the end of Gaiden in a flashback leading up to Sarada's birth (Which he wanted to show), making a total of like 20 chapters. It would seem out of place if we get that now timeline wise, but certainly something I'd like to see.
Expectedly, there are a lot fan-comics and doujins of this around, some are quite good.